The present invention concerns the field of optical monitoring of integrated circuit fabrication.
During the fabrication of integrated circuits, a desired circuit pattern for a given layer of the integrated circuit is etched into a dielectric film. To accomplish this, a photoresist material is disposed on the area of the dielectric film where etching is not desired. An etching gas, which is chemically reactive with the dielectric material and less chemically reactive with the photoresist, is generated in a plasma. The plasma is then supplied to the surface of the dielectric being etched, causing the etching gas to diffuse into the surface of the dielectric film. The etching gas chemically reacts with the dielectric film to form a volatile by-product. The volatile by-product is then desorbed from the surface of the dielectric film and diffuses into the bulk of the etching gas.
After the pattern is etched into the dielectric layer, the photoresist that was used to define the metal circuit pattern on the dielectric layer is removed. Any post-etch residues including sidewall polymer deposition also must be thoroughly removed or stripped from the underlying layer. One dry process used to strip photoresist and photoresist residues from the dielectric layer is known as ashing. The process of ashing is similar to the etching process. The gas used for ashing, however, is more chemically reactive with the photoresist than with-the dielectric. The ashing gas chemically reacts with the photoresist to form a volatile by-product. The volatile by-product diffuses into the bulk of the ashing gas. After the ashing process is complete, the etched pattern is filled with copper or other conductive material.
Optical emission spectroscopy has previously been used to determine the end point of the etching process by providing information about the etching gas and the by-product of the etching gas and dielectric material. The technique relies on the change in the emission intensity of characteristic optical radiation from the dielectric by-product in the plasma. Excited atoms or molecules emit light when electrons relax from a higher energy state to, a lower energy state. Atoms and molecules of different chemical compounds emit a series of unique spectral lines. The emission intensity for each chemical compound within the plasma depends on the relative concentration of the chemical compound in the plasma. A typical optical emission spectroscopy apparatus operates by measuring the emission intensities of the reactive etching gas and the by-product of the etching gas and the dielectric. The emission of the by-product decreases and finally stops when an endpoint is reached. The optical emission spectroscopy apparatus senses the declining emission intensity of the by-product to determine this endpoint.
It is very important to accurately determine the endpoint of stripping, etching, or residue-removal processing of wafers for semiconductor devices. Accurate endpoint detection improves throughput and minimizes damage to other wafer layers. Over-ashing and under-ashing produce undesirable patterns in the integrated circuit wafer. It is difficult to accurately determine endpoint, because process chambers for semiconductor wafer processing offer very little diagnostic access. Optical emission spectroscopy (OES) has been used to determine endpoint measurements, but is inaccurate because of poor optical access to the region of the wafer. If radiant wafer heating lamps are used to promote the ashing process, the light used to heat the wafer interferes with the light emitted at the wafer surface, used to determine endpoint.
In a radiantly heated wafer process chamber the lamps used to heat the wafer emit broadband, blackbody light. The intensity of the broadband, black-body lamp emission may be several orders of magnitude greater than the emission by the reactant by-product of the coating and the plasma being monitored to detect end-point. The broadband, black-body light is most intense during a ramp phase when the lamps are on at full power. The stray light from the lamps becomes a critical issue during ramp phase. Both stray light outside the spectrometer but associated with the optics leading into the spectrometer and stray light which makes its way into the spectrometer make monitoring of the reactant by-product to determine endpoint more difficult. Stray light enters the optics monitoring system, because dirt and coating films, deposited on the optics during the ashing process, and imperfections in the optics scatter stray light into the spectrometer. Additionally, the far wall reflects and emits stray light into the field of view of the optics. The light emitted by the volatile by-product is in part reflected and diffused out of the field of view of the spectrometer by the coating on the optics and optics imperfections. Unwanted lamp light is in part diverted into the field of view of the spectrometer by the coating and dirt on the optics and imperfections in the optics. Coating and dirt on the optics reduces the by-product light, which reaches the spectrometer input and scatters lamp light into the spectrometer input. The increase in lamp light and decrease in volatile by-product light which enter the spectrometer result in a reduced signal-to-noise or signal-to-background ratio, which degrades the performance of the optical detection system.
Spectrometers are designed to direct light into a particular path for each wavelength for measurement. To accomplish this, spectrometers include numerous internal surfaces, off which the light either reflects, refracts, or is transmitted. These internal surfaces have surface imperfections and become dirty causing a small percentage of the lamp light, which enters the spectrometer, to be diffusely scattered. When lamps are used to radiantly heat the wafer a small percent of the high intensity lamp light leads to a moderately high level of randomly scattered light inside the spectrometer. Some lamp light, although at wavelengths other than the wavelength of the light emitted by the by-product, enters wavelength channels of the spectrometer designed to measure light emitted by the volatile by-product. Stray light inside the spectrometer is background or noise, scattered off the imperfect interior surface of the spectrometer. The stray light in the spectrometer results in a reduced signal-to-noise or signal-to-background ratio.
There are special problems with downstream process chambers with regard to the number of available OES signals and accessing the volume from which maximum signal is produced by the ashing and etching reactions. Often, there is both broadband light from the radiant heating lamps and the plasma itself which varies during the ashing and the etching processes, which make it more difficult to measure the light emitted by the wafer. Special cases of low ash rate processes, resulting in weak signals, also make endpoint detection more difficult.
The design of a standard etcher is distinct from the design of a downstream asher in two respects. First, the pressures associated with etchers are much lower. The lower pressure of etchers allows molecular flow, where given molecules of the reactant by-product move freely around the process chamber and bounce off numerous chamber walls many times, creating a substantially uniform distribution of by-product within the reaction chamber. Secondly, the plasma fills the process chamber and remains in the process chamber. For these two reasons, there is often nearly uniform signal strength from the reactant by-product in etchers.
The pressure in a downstream plasma asher is increased. Applicants have observed a transitional region of laminar viscous flow slightly above the wafer surface due to the increase in pressure. The reactant by-product of the coating being removed and the plasma is contained near the wafer surface by the laminar flow. The by-product being monitored is formed on or near the surface of the wafer and is constrained in this region by the laminar flow of the plasma. The region of a downstream plasma asher which is useful in monitoring by-product signal is the region extending from the wafer surface to slightly above the wafer surface.
The present invention is directed to an optically monitored wafer processing system that includes a wafer processing chamber, a wafer treatment apparatus and receiving optics. The wafer processing chamber includes a support for positioning one or more wafers. The wafer treatment apparatus includes means for routing wafer treatment material, such as a plasma, into the processing chamber for removing coating from one or more wafers. The receiving optics are mounted to the chamber in al position to monitor concentrations of the reactant by-product of the coating and the wafer treatment material on or above the surface of the wafer. The optics measure the reactant by-product on or above the surface of the wafer, where the by-product is concentrated.
The optically monitored wafer processing system may include a viewing dump. The viewing dump may be mounted, in the interior of the wafer processing chamber or to the exterior of the wafer processing chamber, adjacent to a window. In one embodiment of the invention, the viewing dump may comprise a narrow port or slot in the chamber wall opposite the optics, a blackened cover and a diamond shaped deflector within the box. The black box and deflector intercept background light within the plasma reaction chamber. The viewing dump increases the signal to background and signal to noise ratios of signals generated by the wafer processing system. Unwanted light from the wafer processing chamber is absorbed by the viewing dump rather than being detected by the optics monitoring system.
In one embodiment of the invention, a polarizer be mounted to the wafer processing chamber to filter signals emitted by the reactant by-product of the coating and the wafer treatment material. The polarizer removes background light and to a lesser extent removes the signal light of the by-product. Although the polarizer reduces desired signal, it has the benefit of increasing the signal to background and signal to noise ratios, because background and noise are removed to a much greater extent.
The receiving optics of the optically monitored wafer processing system can be configured in a linear array. Configuring the receiving optics in a linear array allows each fiber optic fiber of the array to be located near the surface of the wafer and gather light from points on and near the surface of the wafer. In this configuration, the entire line of sight of each fiber optic fiber is near the surface of the wafer. Configuring the receiving optics into a linear array maximizes the desired signal, because the line of sight of each fiber optic fiber extends through a region where the desired signal is concentrated and the background and noise are minimized.
In the preferred embodiment, the optics monitoring system includes a low pass filter. The low pass filter is placed in front of the lens, between the lens and the fiber optic fibers or between the fiber optic fibers and the spectrometer. The low pass filter selectively prevents light from passing through it to the lens, fiber optic fibers or the spectrometer. A low pass filter may be selected, which prevents light with wavelengths other than the wavelength of light emitted by the by-product of coating and plasma.
The receiving optics may also be configured in a fan shaped array. As with the linear array, each fiber optic fiber is located hear the surface of the wafer and gathers light from a point near the surface of the wafer. In the fan shaped array, the lines of sight of the receiving optics cover a greater area above the wafer. By monitoring a greater area above the wafer, a more statistically accurate signal is gathered.
The receiving optics may be arranged to monitor a uniform distribution of points on the surface of the wafer. The receiving optics may be held by a holding block with an orifice to position each optic at an appropriate distance and tangle from a receiving lens to define focal points uniformly distributed about the surface of the wafer. By monitoring a uniform distribution of points on the surface of the wafer, a larger area of the wafer is monitored, which assures more uniform endpoint detection.
These and other advantages and features of the invention will become better understood from the following detailed description of an exemplary embodiment which is described in conjunction with the drawings.